The following publications are believed to represent the current state of the art:    U.S. Pat. Nos. 4,195,913 and 5,153,780;    U.S. Published Patent Application No.: 2009/0065045;    U.S. patent application Ser. No. 12/677,208;
Concentrators employing spherical concave reflective elements suitable for photovoltaic power generation are discussed by Authier, B. and Hill, L., 1980, “High Concentration Solar Collector of the Stepped Spherical Type: Optical Design Characteristics,” Applied Optics, Vol 19, No. 20, pp. 3554-3561;
Concentrators designed for photovoltaic applications are discussed by Kurzweg, U. H., 1980, “Characteristics of Axicon Concentrators for Use in Photovoltaic Energy Conversion”, Solar Energy, Vol. 24, pp. 411-412;
Swanson, R. M., July 1988, “Photovoltaic Dish Solar-Electric Generator”, Proceedings of the Joint Crystalline Cell Research, and Concentrating Collector Projects Review SAND88-0522, Sandia National Laboratories, Albuquerque, N. Mex., pp. 109-119 discuss a parabolic dish in conjugation with a diffuser.